Desilylation of (Zhairsp;)-alpha;-dimethylphenylsilyl vinyl sulfides with fluoride ion:hairsp;1revised mechanism for phenyl group migration in substrates containing an electron-withdrawing group beta; to the sulfur

摘要

J. Chem. Soc. Perkin Trans. 1 1997 3211 Desilylation of (Z)-middot;-dimethylphenylsilyl vinyl sulfides with fluoride ion 1 revised mechanism for phenyl group migration in substrates containing an electron-withdrawing group lsquor; to the sulfur Bianca Flavia Bonini,* Mauro Comes Franchini Mariafrancesca Fochi Germana Mazzanti and Alfredo Ricci Dipartimento di Chimica Organica lsquo;A. Manginirsquo; dellrsquo;Universitagrave; di Bologna Viale Risorgimento n. 4 I-40136 Bologna Italy A detailed investigation of the desilylation of (Z)-a-dimethylphenylsilyl vinyl sulfides with fluoride ion has shown that in substrates containing an electron-withdrawing group b to the sulfur phenyl group migration occurs as a result of a fluoride ion catalyzed retro-Michael reaction. Introduction a-Silyl vinyl sulfides 2andash;d,3a,b are intriguing species since they can react both as vinylsilanes and vinyl sulfides.4a,b Recently we synthesized a number of these compounds and investigated their chemistry.1,5,6 During our study on the protiodesilylation of the a-silyl vinyl sulfides 1 with fluoride ion,1 we found that desilylation of (Z)-a-trimethylsilyl vinyl sulfides 1 (R3 = Me) can be readily achieved to afford stereoselectively the corresponding (Z)-vinyl sulfides.In contrast desilylation of compounds 1 (R3 = Ph) gave unexpected results. In fact while treatment of 1a with moist TBAF in boiling THF gave the desilylated (Z)-a-vinyl sulfide 2a the reaction of the homologous compound 1b under the same conditions gave 3a arising from migration of the phenyl group from the silicon to the adjacent carbon atom (Scheme 1). The different behaviour of 1a and 1b upon desilylation indicates that structural features are crucial for phenyl group migration.According to these preliminary results 1 we attributed the phenyl group migration in 1b to greater nucleophilicity of its sulfur atom compared with that of 1a. Thus compound 1b can be easily protonated in the b position and this initiates the phenyl group migration.1 In this paper we report the full details of the desilylation of (Z)-a-dimethylphenyl silyl vinyl sulfides and provide a new rationale for the phenyl group migration. Results and discussion In order to investigate in detail the desilylation of (Z)-adimethylphenylsilyl vinyl sulfides 1 we synthesized a variety of compounds 1 R3 = Ph R1 = Et Pri n = 1 2 3 and R2 = an electron-withdrawing group (EWG) an electron-donating R1 S SiMe2R3 R2 1 ( )n Scheme 1 S CO2Et Ph SiMe2Ph S CO2 Et H S CO2 Et H n = 2 TBAF THF H2O heat 2a 3a n = 1 1a n = 2 1b n = 1 ( )n group (EDG) or an alkyl group.Products 1 were prepared from the corresponding (Z)-a-dimethylphenylsilyl enethiols 4 1 with two different procedures illustrated in Scheme 2 and Table 1. The full details of the synthesis of (Z)-1-dimethyl(phenyl)- silylbut-1-enethiol 4a have been previously reported; 1 the (Z)- 1-dimethyl(phenyl)silyl-3-methylbut-1-enethiol 4b has been prepared by a similar procedure to that used to prepare 4a (see Experimental section). According to path A products 1andash;i were obtained in a stereoselective manner by reaction of 4 with halides R2(CH2)nX in acetone in the presence of dry K2CO3 at room temperature; Scheme 2 R X SH R1 SiMe2Ph S R1 SiMe2Ph R2 R S R1 SiMe2Ph R2 1andash;i R2 = EWG EDG alkyl Acetone K2CO3 THF DBU 1b 1jndash;n R2 = EWG 4a R1 = Et 4b R1 = Pri path A path B ( )n ( )n Table 1 Synthesis of (Z)-a-dimethylphenylsilyl vinyl sulfides 1 (paths A and path B) 1 a b b c d e e f g h i j k l m n R1 Et Et Et Et Et Et Et Et Et Et Et Et Et Et Pri Pri n 1 2 1 1 1 1 2 1 2 3 R2 CO2Et CO2Et CO2Et CN COMe OEt OEt OEt H H H CN COMe CO2Me CO2Et CO2Me X I Cl Cl Cl Cl Cl Cl I Br Br Path A A B A A A A A A A B B B B B Yield () b 80 a 80 a 80 89 69 20 c 90d 26 95 a 90 80 91 66 92 84 86 a Ref 1.b The yields were determined after chromatography. c Beside 1e product 5 was obtained in 25 yield. d Prepared using triethylamine as the base (see text). 3212 J. Chem. Soc. Perkin Trans. 1 1997 good yields were generally obtained except in the case of halides containing an ethoxy group in the a- or in a b-position.During the synthesis of 1e (20 yield) the thioacetal 5 was formed in 25 yield arising probably by an acid-catalyzed reaction 7 (HCl) of the O,S-thioacetal 1e with the starting enethiols 4a (Scheme 3). The structure of product 5 was established on the basis of analytical and spectral data (see Experimental section). In an effort to improve the yield of products 1e and 1f we allowed 4a to react with chloromethyl ethyl ether in diethyl ether in the presence of an equimolar amount of triethylamine to give product 1e (90) yield; attempted reaction of 4a with chloroethyl ethyl ether gave under similar reaction conditions recovery of 4a. By path B (Scheme 2) the enethiols 4 gave a base- (DBU) catalyzed Michael type addition 8 with olefins bearing an electron-withdrawing group to afford (Z)-a-dimethylphenylsilyl vinyl sulfides 1b 1jndash;n in very good yields (Table 1).Compounds 1andash;n were desilylated by reaction with TBAF in moist THF at reflux. Normal protiodesilylation occurs with the following substrates containing an electron-withdrawing group a to the sulfur 1a c d; substrates containing an electrondonating group a or b to the sulfur 1e f; and substrates with an alkyl group bonded to the sulfur 1gndash;i (Scheme 4). The yield of products 2 were generally good (Table 2) except for 2g where the product was volatile. Desilylation of 1d gave beside product 2d a cyclic compound 6 (18) arising from an intramolecular cyclization of the intermediate vinyl anion formed during the desilylation on the carbonyl group.In contrast the derivatives containing an electron-withdrawing group b to the sulfur 1b jndash;n gave upon treatment with moist TBAF in boiling THF products 3 the result of phenyl group migration (Scheme 5 Table 3). These results clearly show the inadequacy of our previous Scheme 3 Et S Si OEt Et Si S S Si Et 5 1e + 4a Si = SiMe2Ph Scheme 4 S SiMe2Ph R2 S H H R2 2a cndash;i THF H2O heat TBAF 1a c-i ( )n ( )n S Et Me HO 6 Table 2 Protiodesilylation of (Z)-a-dimethylphenylsilyl vinyl sulfides 1 1 a c d e f g h i n 1 1 1 1 2 1 2 3 R2 CO2Et CN COMe OEt OEt H H H 2 a c d e f g h i Yield () 80a 72 82 b 95 98 46 c 98 75 a Compared with an authentic sample (ref. 1). b Beside product 2d a cyclic product 6 was obtained in 18 yield (see Experimental section).c Ref. 1. mechanism.1 In fact substrates containing an electrondonating group in a position a or b to the sulfur which would increase the availability of electron density on the sulfur did not favour phenyl group migration. A plausible mechanistic interpretation of the phenyl group migration is outlined in Scheme 6. Substrates 1 containing a hydrogen atom a to an electronwithdrawing group can be easily deprotonated by a base such as TBAF. In fact TBAF behaves not only as a potent source of nucleophilic fluoride but also as a potent base.9 If the produced carbanion can undergo a retro-Michael reaction as in the case of the intermediate 7 extrusion of the olefins 8 takes place to give the enethiolate 9. The silicon may or may not have a fluoride ion coordinated to it in these stages.The phenyl group in the thione 10 can then migrate from the silicon to the adjacent carbon to give 11; this by a Michael addition to the olefin 8 affords after desilylation product 3. To test this hypothesis we desilylated 1b in the presence of 1 equiv. of methyl acrylate to obtain compounds 3b and 3l; these arose from the thiolate 11a (R1 = Et) as a result of Michael addition with ethyl acrylate and methyl acrylate followed by desilylation (Scheme 7). A further proof of the mechanism depicted in Scheme 6 was obtained by desilylation of equimolar amounts of 1b and 1n under the usual experimental conditions when four crossed products 3b 3l 3n and 3m were obtained in the yields reported in Scheme 8. These yields were calculated from the 1H NMR spectrum of the crude mixture by comparison with the 1H NMR spectra of authentic samples (see Scheme 5 R1 S SiMe2Ph R2 R1 S R2 Ph 3b jndash;n 1b jndash;n THF H2O heat TBAF R2 = EWG Scheme 6 R1 Ph S EWG EWG R1 Sndash; SiMe2F Ph S Si R1 Me Ph Me S SiMe2Ph R1 R1 Sndash; SiMe2Ph EWG R1 S SiMe2Ph EWG R1 S SiMe2Ph EWG Fndash; Fndash; Fndash; + SiMe2F2 1b jndash;n 7 9 10 11 3 ndash; 8 8 H+/Fndash; Table 3 1 b j k l m n R1 Et Et Et Et Pri Pri R2 CO2Et CN COMe CO2Me CO2Et CO2Me 3 b j k l m n Yield () 48 a 48 48 49 60 48 a Ref.1. J. Chem. Soc. Perkin Trans. 1 1997 3213 Experimental section). The four products arise from the reaction of the two intermediates 11a (R1 = Et in Scheme 6) and 11b (R1 = Pri in Scheme 6) with ethyl acrylate and methyl acrylate which are formed in the retro-Michael reaction stage. We then examined the desilylation of the enethiol 4a which was expected to give the same intermediate 11a.In fact treatment of 4a with TBAF in boiling THF gave an inseparable mixture of the disulfide 12 arising from dimerization of the thiolate 11a followed by desilylation and of product 13 (Scheme 9). The formation of product 13 is probably the result of alkylation of the thiolate 11a (R1 = Et in Scheme 6) either by the TBAF itself or by other species deriving from its decomposition at higher temperature.10 Product 12 could be obtained pure by performing the desilylation of 4a with CsF in boiling Me3CN (Scheme 9); its structure was established on the basis of satisfactory analytical and spectral results. Conclusion In summary we have examined the desilylation of (Z)-adimethylphenylsilyl vinyl sulfides 1 with fluoride ion and rationalized the migration of the phenyl group from the silicon to the adjacent carbon atom in substrates containing an electron-withdrawing group b to the sulfur.Substrates containing an electron-withdrawing group a to the sulfur or an electron-donating group a or b to the sulfur or an alkyl chain bonded to the sulfur gave compounds arising from normal protiodesilylation. Experimental Bps and mps are uncorrected. 1H NMR and 13C NMR spectra Scheme 7 S SiMe2Ph CO2Et Ph S CO2Me CO2Me Ph S CO2Et 1b 3l 3b THF H2O heat TBAF 25 25 Scheme 8 S SiMe2Ph CO2Et Ph S CO2Me Ph S CO2Et S SiMe2Ph CO2Me Ph S CO2Et Ph S CO2Me 13 30 7.5 30 Yield () THF H2O heat TBAF + 1b 1n 3b 3l 3n 3m Scheme 9 Ph S Ph S Ph S Bu SH SiMe2Ph THF H2O heat TBAF CsF + 12 13 12 4a MeCN H2O heat )2 )2 were recorded with Varian Gemini 200 or 300 MHz spectrometers as solutions in CDCl3 chemical shifts (d) are given in ppm relative to tetramethylsilane TMS.J Values are given in Hz. 13C NMR spectra assignments were made by DEPT experiments. Mass spectra were obtained using a VG 7070-E (EI 70 eV) spectrometer. IR spectra were recorded on a Perkin- Elmer model 257 grating spectrometer. Reactions were conducted in oven-dried (120 8C) glassware under a positive argon atmosphere. Transfer of anhydrous solvents or mixtures was accomplished with oven-dried syringes. THF was distilled from sodium benzophenone just prior to use and stored under argon. Et2O was distilled from phosphorus pentoxide. The reactions were monitored by TLC performed on silica gel plates (Baker- flex IB2-F). Column chromatography was performed with Merck silica gel 60 (70ndash;230 mesh) and preparative thick layer chromatography was carried out on glass plates using a 1-mm layer of Merck silica gel 60 Pf254 or aluminium oxide F254.Light petroleum refers to the fraction with bp 40ndash;60 8C. Because of the small scale used in the preparations new compounds which were oily products have been characterized by accurate mass measurements. All chemicals were used as obtained or purified by distillation as needed. Sodium hydrogen carbonate 99 was purchased from Carlo Erba Reagenti; hydrogen chloride was purchased from Praxair (Belgium). 3-Methylbutanoyl(dimethyl)phenylsilane 3-Methylbutanoyl chloride (1.20 g 1.22 cm3 10.0 mmol) in anhydrous THF (3 cm3) was added slowly to lithium bis- dimethyl(phenyl)silylcuprate 11 (10.0 mmol) at 278 8C under argon.The mixture was stirred at 278 8C for 1 h and then allowed to warm to 0 8C at which temperature it was stirred for 1 h. The mixture was then treated with saturated aqueous ammonium chloride to quench the reaction and extracted with diethyl ether. The extract was dried and concentrated under reduced pressure. Chromatography of the residue on silica gel column light petroleumndash;diethyl ether (10 1) as eluent gave as the higher RF fraction a product arising from the silylcuprate and as the lower RF fraction the acylsilane (1.6 g 73) as a yellow oil (Found M1 220.1379. C13H20OSi requires M 220.1283); nmax(CCl4)/cm21 1650 (CO) 1430 1110 (SiPh) and 1250 (SiMe2); dH(CDCl3) 0.45 (6H s SiMe2) 0.85 (6H d J 6.6 CH3) 2.2 (2H m CH) 2.50 (2H d J 6.7 CH2CO) 7.45ndash;7.50 (3H m ArH) and 7.58ndash;7.62 (2H m ArH); dC(CDCl3) 24.86 (SiMe2) 22.54 (2CH3) 22.86 (CH) 57.58 (CH2CO) 128.01 129.71 133.85 (ArCH) 139.15 (ArC) and 246.51 (CO); m/z (EI) 220 (M1) 163 M1 2 (CH3)2CHCH2 and 135 (SiMe2Ph).(Z)-1-Dimethyl(phenyl)silyl-3-methylbut-1-enethiol 4b Hydrogen chloride and hydrogen sulfide were bubbled into a solution of 3-methylbutanoyl(dimethyl)phenylsilane (0.35 g 1.59 mmol) in diethyl ether (70 cm3) at 220 8C until the starting ketone had disappeared TLC with light petroleumndash;diethyl ether (10 1) as eluent. After the mixture has been allowed to warm to room temperature it was treated with solid sodium hydrogen carbonate until evolution of carbon dioxide ceased; it was then left overnight. Filtration and concentration of the mixture under reduced pressure gave the pure (Z)-enethiol 4b as an oil (0.35 g 1.48 mmol 93) (Found M1 236.1059.C13H20SSi requires M 236.1055); nmax(CCl4)/cm21 2540 (SH) 1430 1110 (SiPh) 1250 and 890 (SiMe2); dH(CDCl3) 0.45 (6H s SiMe2) 1.05 (6H d J 10.0 2CH3) 2.40 (1H s SH) 2.65ndash;2.85 (1H m CH) 5.80 (1H d J 9.0 vinylic H) 7.40 (3H m ArH) and 7.70 (2H m ArH); dC(CDCl3) 23.16 (SiMe2) 21.82 (2CH3) 29.89 (CH) 125.24 (C) 127.94 129.44 134.18 (ArCH) 136.57 (C) and 147.52 (vinylic CH); m/z (EI) 236 (M1) 221 (M1 2 CH3) 187 (M1 2 H2S) 158 (M1 2 C6H6) 143 (M1 2 C7H9) and 135 (SiMe2Ph). Synthesis of (Z)-middot;-silyl vinyl sulfides 1 general method (Path A). To a solution of (Z)-a-silyl enethiol 4 (1.0 mmol) in acetone (4 cm3) solid oven-dried K2CO3 (1.3 mmol) and the halide (1.1 3214 J. Chem. Soc. Perkin Trans. 1 1997 mmol) were added.The mixture was stirred at room temperature until the starting enethiol had disappeared. The mixture was then diluted with water and extracted with diethyl ether. The extract was dried and concentrated to give the title product. In some cases the product was purified by chromatography on silica light petroleumndash;diethyl ether (10 1) as eluent. (Z)-{1-Dimethyl(phenyl)silylbut-1-enylsulfanyl}acetonitrile 1c. Yield 89 as an oil (Found M1 261.1012. C14H19NSSi requires M 261.1007); nmax(CCl4)/cm21 2242 (CN) 1426 (SiPh) 1248 (SiMe2) and 1109 (SiPh); dH(CDCl3) 0.50 (6H s SiMe2) 1.05 (3H t J 7.5 CH3) 2.55 (2H m CH2) 2.85 (2H s CH2CN) 6.65 (1H t J 6.9 vinylic H) 7.38 (3H m ArH) and 7.60 (2H m ArH); dC(CDCl3) 22.90 (SiMe2) 13.50 (CH3) 19.55 24.45 (CH2) 116.65 (CN) 128.01 129.58 133.93 (ArCH) 135.10 (ArC) 136.90 (vinylic C) and 158.73 (vinylic CH); m/z (EI) 261 (M1) 260 (M1 2 1) 246 (M1 2 CH3) 221 (M1 2 CH2CN) and 135 (SiMe2Ph).(Z)-{1-Dimethyl(phenyl)silylbut-1-enylsulfanyl}propan-2- one 1d. Chromatography on silica gel of the crude reaction mixture using light petroleumndash;diethyl ether (8 1) as eluent gave the title product as oil (70) (Found M1 278.1158. C15H22- OSSi requires M 278.1161); nmax(CCl4)/cm21 1720 (CO) 1440 (SiPh) 1240 (SiMe2) and 1110 (SiPh); dH(CDCl3) 0.45 (6H s SiMe2) 1.00 (3H t J 7.5 CH3) 2.00 (3H s COCH3) 2.40 (2H m CH2) 3.10 (2H s CH2CO) 6.35 (1H t J 6.8 vinylic H) 7.35 (3H m ArH) and 7.65 (2H m ArH); dC(CDCl3) 22.49 (SiMe2) 13.41 (CH3) 24.32 (CH2) 27.96 (CH3) 44.57 (SCH2) 127.82 129.25 133.98 (ArCH) 131.15 (ArC) 137.62 (vinylic C) 154.73 (vinylic CH) and 203.18 (CO); m/z (EI) 278 (M1) 263 (M1 2 CH3) and 135 (SiMe2Ph).(Z)-1-Dimethyl(phenyl)silyl-1-ethoxymethylsulfanylbut-1- ene 1e. Chromatography on alumina of the crude product using light petroleumndash;diethyl ether (20 1) as eluent gave as the higher RF fraction product 5 as an oil (25) (Found M1 456.1792. C25H36S2Si2 requires M 456.1797); nmax(CCl4)/cm21 1430 1110 (SiPh) and 1250 (SiMe2); dH(CDCl3) 0.40 (12H s 2 SiMe2) 1.00 (6H t J 7.5 2 CH3) 2.40 (4H m 2CH2) 3.4 (2H s SCH2S) 6.30 (2H t J 6.8 2 vinylic H) 7.4 (6H m ArH) and 7.6 (4H m ArH); dC(CDCl3) 22.35 (SiMe2) 13.39 (CH3) 24.37 40.03 (CH2) 127.74 129.10 134.00 (ArCH) 132.72 (ArC) 137.86 (vinylic C) and 153.77 (vinylic CH); m/z (EI) 456 (M1) 221 (CH3CH2CHCSSiMe2Ph) 144 (CH3CH2- CHCSCH2SC) 135 (SiMe2Ph) and 91 (C7H7); as the second RF fraction the title product as an oil (20) (Found M1 280.1315.C15H24OSSi requires M 280.1317); nmax(CCl4)/cm21 1080 1140 (C-O-C) 1430 1110 (SiPh) and 1250 (SiMe2); dH(CDCl3) 0.45 (6H s SiMe2) 1.05 (3H t J 7.6 CH3) 1.15 (3H t J 7.1 CH3) 2.45 (2H m CH2) 3.5 (2H q J 7.1 OCH2) 4.45 (2H s CH2O) 6.35 (1H t J 6.8 vinylic H) 7.35 (3H m ArH) and 7.65 (2H m ArH); dC(CDCl3) 22.55 (SiMe2) 13.39 14.70 (CH3) 24.32 63.99 75.91 (CH2) 127.68 129.06 134.08 (ArCH) 131.89 (ArC) 139.00 (vinylic C) and 152.42 (vinylic CH); m/z (EI) 280 (M1) 221 (M1 2 CH2OC2H5) and 135 (SiMe2Ph). Improved procedure for 1e. To a solution of (Z)-1-dimethyl- (phenyl)silylbut-1-enethiol (0.13 g 0.59 mmol) in anhydrous diethyl ether (3 cm3) chloromethyl ethyl ether (0.065 g 0.07 cm3 0.7 mmol) and triethylamine (0.07 g 0.1 cm3 0.7 mmol) were added.The mixture was stirred at room temperature until the starting enethiol had disappeared TLC light petroleumndash; diethyl ether (20 1) as eluent 10 min.. The mixture was then diluted with water and extracted with diethyl ether. The organic layer was dried and concentrated to give the title product as an oil (1.15 g 0.53 mmol 90). Use of an excess of chloromethyl ethyl ether and triethylamine gave the title product in quantitative yield. (Z)-1-Dimethyl(phenyl)silyl-1-(29-ethoxyethylsulfanyl)but-1- ene 1f. Chromatography on silica gel of the crude product using light petroleumndash;diethyl ether (10 1) as eluent gave the title product as an oil (26) (Found M1 294.1478. C16H26OSSi requires M 294.1474); nmax(CCl4)/cm21 1430 1110 (SiPh) 1248 (SiMe2) and 1150ndash;1070 (C-O-C); dH(CDCl3) 0.45 (6H s SiMe2) 1.05 (3H t J 7.6 CH3) 1.15 (3H t J 7.1 CH3) 2.45 (2H m CH2) 2.62 (2H t J 7.2 SCH2) 3.30 (2H t J 7.2 CH2) 3.34 (2H q J 7.1 CH2O) 6.3 (1H t J 6.8 vinylic H) 7.35 (3H m ArH) and 7.45 (2H m ArH); dC(CDCl3) 22.37 (SiMe2) 13.37 15.11 (CH3) 24.22 33.64 66.05 69.66 (CH2) 127.73 129.07 133.95 (ArCH) 132.20 (ArC) 138.60 (vinylic C) and 152.90 (vinylic CH); m/z (EI) 294 (M1) 249 (M1 2 OC2H5) 221 (M1 2 CH2CH2OC2H5) 179 (221 2 C3H6) and 135 (SiMe2Ph).A reaction performed using triethylamine as a base in diethyl ether with the (Z)-enethiol 4a gave recovery of starting material. (Z)-1-Dimethyl(phenyl)silyl-1-ethylsulfanylbut-1-ene 1h. Yield 90 as an oil (Found M1 250.1216. C14H22SSi requires M 250.121 15); nmax(CCl4)/cm21 1430 (SiPh) 1250 (SiMe2) and 1100 (SiPh); dH(CDCl3) 0.50 (6H s SiMe2) 1.03 (3H t J 7.4 CH3) 1.06 (3H t J 7.5 2CH3) 2.45 (4H m 2CH2) 6.30 (1H t J 6.5 vinylic H) 7.38 (3H m ArH) and 7.60 (2H m ArH); dC(CDCl3) 22.30 (SiMe2) 13.39 14.81 (CH3) 24.14 28.42 (CH2) 127.66 128.99 133.93 (ArCH) 132.80 (ArC) 138.05 (vinylic C) and 152.194 (vinylic CH); m/z (EI) 250 (M1) 221 (M1 2 C2H5) and 135 (SiMe2Ph).(Z)-1-Dimethyl(phenyl)silyl-1-propylsulfanylbut-1-ene 1i. Yield 80 as an oil (Found M1 264.1362. C15H24SSi requires M 264.1368); nmax(CCl4)/cm21 1426 (SiPh) 1246 (SiMe2) and 1108 (SiPh); dH(CDCl3) 0.40 (6H s SiMe2) 0.80 (3H t J 7.3 CH3) 1.00 (3H t J 7.6 CH3) 1.40 (2H m CH2) 2.40 (4H m 2CH2) 6.25 (1H t J 6.7 vinylic H) 7.35 (3H m ArH) and 7.65 (2H m ArH); dC(CDCl3) 22.32 (SiMe2) 13.24 13.36 (CH3) 23.22 24.12 36.37 (CH2) 127.61 128.93 133.87 (ArCH) 134.60 (ArC) 138.22 (vinylic C) and 152.03 (vinylic CH); m/z (EI) 264 (M1) 221 (M1 2 C3H7) and 135 (SiMe2- Ph).Synthesis of (Z)-middot;-silyl vinyl sulfides 1 general method (Path B) To a solution of (Z)-a-silyl enethiol (1.0 mmol) in THF (5 cm3) 1,8-diazabicyclo5.4.0undec-7-ene (DBU) (1.1 mmol) and the olefin (1.1 mmol) were added. The mixture was stirred at room temperature until the starting enethiol had disappeared. The mixture was then treated with water and extracted with diethyl ether. The extract was dried and concentrated to give the title product. All the yields were calculated on the crude products. The crude products were used for further reactions without purification. In some cases the products were purified by chromatography on silica light petroleumndash;diethyl ether (10 1) as eluent for the complete characterization analysis.3-{(Z)-1-Dimethyl(phenyl)silylbut-1-enylsulfanyl}propionitrile 1j. Yield 91 as an oil (Found M1 275.1168. C15H21 NSSi requires M 275.1164); nmax(CCl4)/cm21 2248 (CN) 1426 1109 (SiPh) and 1248 (SiMe2); dH(CDCl3) 0.45 (6H s SiMe2) 1.05 (3H t J 7.6 CH3) 2.15 (2H t J 7.2 CH2) 2.45 (2H m CH2) 2.55 (2H t J 7.2 CH2) 6.45 (1H t J 6.8 vinylic H) 7.35 (3H m ArH) and 7.65 (2H m ArH); dC(CDCl3) 22.68 (SiMe2) 13.38 (CH3) 18.22 24.34 29.36 (CH2) 118.20 (CN) 128.01 129.50 133.81 (ArCH) 131.12 (ArC) 137.65 (vinylic C) and 155.03 (vinylic CH); m/z (EI) 275 (M1) 260 (M1 2 CH3) 246 (M1 2 C2H5) 221 (M1 2 CH2CH2CN) and 135 (SiMe2Ph). 4-{(Z)-1-Dimethyl(phenyl)silylbut-1-enylsulfanyl}butan-2- one 1k.Chromatography on silica gel of the crude product using light petroleumndash;diethyl ether (10 1) as eluent gave the title product as an oil (66) (Found M1 292.1319. C16H24OSSi requires M 292.1317); nmax(CCl4)/cm21 1725 (COMe) 1430 1110 (SiPh) and 1250 (SiMe2); dH(CDCl3) 0.45 (6H s SiMe2) 1.00 (3H t J 7.6 CH3) 1.95 (3H s COCH3) 2.3ndash;2.4 (4H m 2CH2) 2.6 (2H t J 7.2 CH2) 6.3 (1H t J 6.7 vinylic H) 7.35 (3H m ArH) and 7.65 (2H m ArH); dC(CDCl3) 22.55 (SiMe2) 13.38 (CH3) 24.21 27.98 (CH2) 29.68 (COCH3) 43.68 (CH2) 127.74 129.11 133.90 (ArCH) 132.46 (ArC), J. Chem. Soc. Perkin Trans. 1 1997 3215 138.02 (vinylic C) 152.85 (vinylic CH) and 206.74 (CO); m/z (EI) 292 (M1) 277 (M1 2 CH3) 221 (M1 2 CH2CH2COMe) 189 (M1 2 SCH2CH2COMe) and 135 (SiMe2Ph). Ethyl 3-{(Z)-1-dimethyl(phenyl)silylbut-1-enylsulfanyl}propionate 1b.Yield 80 the product was compared with an authentic sample.1 Methyl 3-{(Z)-1-dimethyl(phenyl)silylbut-1-enylsulfanyl}- propionate 1l. Yield 92 as an oil (Found M1 308.1264. C16H24O2SSi requires M 308.1266); nmax(CCl4)/cm21 1745 (CO2Me) 1435 1115 (SiPh) and 1250 (SiMe2); dH(CDCl3) 0.40 (6H s SiMe2) 0.95 (3H t J 7.6 CH3) 2.29 (2H t J 7.1 CH2) 2.38 (2H m CH2) 2.62 (2H t J 7.7 CH2) 3.60 (3H s OCH3) 6.30 (1H t J 6.6 vinylic H) 7.30 (3H m ArH) and 7.55 (2H m ArH); dC(CDCl3) 22.48 (SiMe2) 13.37 (CH3) 24.23 29.03 34.50 (CH2) 51.60 (OCH3) 127.76 129.11 133.90 (ArCH) 131.93 (ArC) 137.92 (vinylic C) 153.67 (vinylic CH) and 172.28 (CO); m/z (EI) 308 (M1) 222 (M1 2 CHCH2CO2Me) and 135 (SiMe2Ph). Ethyl 3-{(Z)-1-dimethyl(phenyl)silyl-3-methylbut-1-enylsulfanyl} propionate 1m.Yield 84 as an oil (Found M1 336.1573. C18H28O2SSi requires M 336.1579); nmax(CCl4)/cm21 1740 (CO2Et) 1435 1115 (SiPh) and 1250 (SiMe2); dH(CDCl3) 0.45 (6H s SiMe2) 1.00 (6H d J 6.7 2CH3) 1.20 (3H t J 7.3 CH3) 2.30 (2H t J 7.5 CH2) 2.60 (2H t J 7.5 CH2) 3.15 (1H m CH) 4.09 (2H q J 7.3 OCH2) 6.15 (1H d J 8.9 vinylic H) 7.35 (3H m ArH) and 7.55 (2H m ArH); dC(CDCl3) 22.27 (SiMe2) 14.16 (CH3) 22.22 (2CH3) 29.11 (CH2) 29.60 (CH) 34.81 (CH2) 60.41 (OCH2) 127.76 129.08 133.84 (ArCH) 129.72 (ArC) 138.13 (vinylic C) 159.15 (vinylic CH) and 171.78 (CO); m/z (EI) 336 (M1) 235 (M1 2 CH2CH2- CO2Et) and 135 (SiMe2Ph). Methyl 3-{(Z)-1-dimethyl(phenyl)silyl-3-methylbut-1-enylsulfanyl} propionate 1n. Yield 86 as an oil (Found M1 322.1426. C17H26O2SSi requires M 322.1423); nmax(CCl4)/cm21 1750 (CO2Me) 1435 1115 (SiPh) and 1250 (SiMe2); dH(CDCl3) 0.45 (6H s SiMe2) 1.00 (6H d J 6.6 2CH3) 2.30 (2H t J 7.4 CH2) 2.65 (2H t J 7.4 CH2) 3.20 (1H m CH) 3.62 (3H s OCH3) 6.20 (1H d J 9.1 vinylic H) 7.35 (3H m ArH) and 7.65 (2H m ArH); dC(CDCl3) 22.30 (SiMe2) 22.18 (2 CH3) 29.04 (CH2) 29.58 (CH) 34.55 (CH2) 51.49 (OCH3) 127.74 129.05 133.81 (ArCH) 129.85 (ArC) 138.06 (vinylic C) 159.13 (vinylic CH) and 172.13 (CO); m/z (EI) 322 (M1) 235 (M1 2 CH2CH2CO2Me) and 135 (SiMe2Ph).General procedure for the desilylation of (Z)-middot;-dimethylphenylsilyl vinyl sulfides 1 A solution of tetrabutylammonium fluoride (TBAF) in THF (1.0 M; 1.1 mmol) was added to a solution of (Z)-a-silyl vinyl sulfides (1 mmol) in moist THF (6 cm3). The reaction mixture was stirred at reflux temperature until the starting a-silyl vinyl sulfide had disappeared after which it was diluted with saturated aqueous ammonium chloride to quench the reaction and extracted with diethyl ether.The extract was washed several times with water and then dried and concentrated under reduced pressure. The crude product was purified by preparative thick layer chromatography to give as the higher RF fraction a product arising from the SiMe2Ph moiety and as the lower RF fraction the desilylated product. (Z)-But-1-enylsulfanylacetonitrile 2c. Chromatography on silica gel of the crude product using light petroleumndash;diethyl ether (10 1) as eluent gave the title product as an oil (72) (Found M1 127.0459. C6H9NS requires M 127.0456); nmax(CCl4)/cm21 2260 (CN); dH(CDCl3) 1.05 (3H t J 7.5 CH3) 2.25 (2H m CH2) 3.4 (2H s CH2CN) 5.94 (1H dt J1 9.2 J2 6.9 vinylic H) and 6.02 (1H dt J1 9.2 J2 1.1 vinylic H); dC(CDCl3) 13.44 (CH3) 18.92 22.65 (CH2) 117.15 (CN) and 119.30 and 137.88 (vinylic CH); m/z (EI) 127 (M1) and 112 (M1 2 CH3).(Z)-But-1-enylsulfanylpropan-2-one 2d. A mixture of two products was obtained as shown by the 1H NMR spectrum of the crude reaction mixture the title product (dH 5.60ndash;5.73 and 5.79ndash;5.85) (82) and a cyclic product 6 (dH 5.60) (18). Since the cyclic product 6 was found to be rather unstable on silica it was characterized in the crude mixture nmax(CCl4/cm21 3500 br (OH); dH(CDCl3) 0.95 (3H t J 7.6 CH3) 1.50 (3H s CH3) 1.85 (2H m CH2) 3.05ndash;3.30 (2H dd J 10 CH2) and 5.60 (1H t J 7.2 vinylic H); dC(CDCl3) 13.44 (CH3) 21.13 (CH2) 26.81 (CH3) 29.80 (C) 41.65 (CH2S) 118.57 (vinylic CH) and 144.80 (vinylic C); m/z (EI) 144 (M1) 129 (M1 2 CH3) and 101 (M1 2 COCH3).Chromatography on silica gel of the crude product using light petroleumndash;dichloromethane (2 1) as eluent gave as the higher RF fraction a product arising from the SiMe2Ph moiety and as the second RF fraction the desilylated product as an oil (65) (Found M1 144.0612. C7H12OS requires M 144.0609); nmax(CCl4)/cm21 1715 (COCH3); dH(CDCl3) 1.05 (3H t J 7.6 CH3) 2.15 (2H m CH2) 2.3 (3H s COCH3) 3.35 (2H s SCH2CO) 5.60ndash;5.73 (1H dt J1 9.3 J2 6.9 vinylic H) and 5.79ndash;5.85 (1H dt J1 9.3 J2 1.2 vinylic H); dC(CDCl3) 13.47 (CH3) 22.46 (CH2) 27.66 (CH3) 43.46 (SCH2CO) 121.68 and 133.87 (vinylic CH) and 203.42 (CO); m/z (EI) 144 (M1) 101 (M1 2 COMe) 87 (M1 2 CH2COMe) 55 (M1 2 SCH2COMe) and 43 (COMe).(Z)-1-Ethoxymethylsulfanylbut-1-ene 2e. Yield 95 as an oil (Found M1 146.0762. C7H14OS requires M 146.0765); nmax(CCl4)/cm21 1070 (C-O-C); dH(CDCl3) 1.0 (3H t J 7.5 CH3) 1.2 (3H t J 7.0 CH3) 2.15 (2H m CH2) 3.6 (2H q J 7.0 OCH2) 4.75 (2H s SCH2O) 5.65 (1H dt J1 9.3 J2 7.1 vinylic H) and 6.05 (1H dt J1 9.3 J2 1.4 vinylic H); dC(CDCl3) 13.41 14.71 (CH3) 22.61 (CH2) 63.79 (OCH2) 74.33 (SCH2O) and 122.37 and 132.35 (vinylic CH); m/z (EI) 146 (M1) 101 (M1 2 OEt) 87 (M1 2 CH2OEt) 59 (CH2OEt) 45 (OEt) and 40 (C3H4). Because of its high volatility an attempt to purify this product lowered its yield. (Z)-1-(29-Ethoxyethylsulfanyl)but-1-ene 2f. Yield 98 as an oil (Found M1 160.0926. C8H16OS requires M 160.0922); nmax(CCl4)/cm21 1070 (C-O-C); dH(CDCl3) 1.0 (3H t J 7.6 CH3) 1.2 (3H t J 7.0 CH3) 2.1 (2H m CH2) 2.8 (2H t J 7.0 CH2) 3.52 (2H q J 7.0 CH2O) 3.60 (2H t J 6.9 OCH2) 5.55 (1H dt J1 9.4 J2 7.0 vinylic H) and 5.9 (1H dt J1 9.43 J2 1.4 vinylic H); dC(CDCl3) 13.44 15.12 (CH3) 22.42 33.13 (CH2) 66.41 70.18 (OCH2) and 123.92 131.70 (vinylic CH); m/z (EI) 160 (M1) 101 (M1 2 CH2CH2OEt) 73 (CH2CH2OEt) 59 (CH2OEt) 45 (OEt) and 40 (C3H4).Because of its high volatility an attempt to purify this product lowered its yield. (Z)-1-Ethylsulfanylbut-1-ene 2h. Yield 98 as an oil (Found M1 116.0657. C6H12S requires M 116.0660); dH(CDCl3) 0.9 (3H t J 7.4 CH3) 1.24 (3H t J 7.2 CH3) 2.1 (2H m CH2) 2.65 (2H q J 7.4 CH2) 5.53 (1H dt J1 10 J2 6.9 vinylic H) and 5.85 (1H dt J1 10 J2 1.4 vinylic H); m/z (EI) 116 (M1) 87 (M1 2 C2H5) 73 (87 2 CH2) and 40 (C3H4). Because of its high volatility an attempt to purify this product lowered the yield.(Z)-1-Propylsulfanylbut-1-ene 2i. Yield 75 as an oil (Found M1 130.0812. C7H14S requires M 130.0816); dH(CDCl3) 0.95 (6H 2t 2CH3) 1.7 (2H m CH2) 2.1 (2H m CH2) 2.6 (2H t J 7.7 SCH2) 5.5 (1H dt J1 9.3 J2 7.1 vinylic H) and 5.84 (1H dt J1 9.3 J2 1.4 vinylic H); m/z (EI) 130 (M1) 87 (M1 2 C3H7) and 40 (C3H4). Because of its high volatility an attempt to purify this product lowered its yield. 3-(19-Phenylbutylsulfanyl)propionitrile 3j. Chromatography on silica gel of the crude product using light petroleumndash;ethyl acetate (7 1) as eluent gave the title product as an oil (48) (Found M1 219.1085. C13H17NS requires M 219.1082); nmax(CCl4)/cm21 2248 (CN); dH(CDCl3) 0.9 (3H t J 7.3 CH3) 1.3 (2H m CH2) 1.8 (2H m CH2) 2.45 (2H t J 7.4 CH2CN) 2.55 (2H t J 7.4 CH2S) 3.90 (1H dd J 7.8 CHPh) and 7.3 (5H m ArCH); dC(CDCl3) 13.76 (CH3) 18.66 20.81 26.58 38.47 (CH2) 50.01 (CH) 118.43 (CN) 127.56 127.86 128.76 (ArCH) and 142.04 (ArC); m/z (EI) 219 (M1) 176 (M1 2 CH2- CH2CH3) 133 (M1 2 SCH2CH2CN) and 91 (C7H7).3216 J. Chem. Soc. Perkin Trans. 1 1997 4-(19-Phenylbutylsulfanyl)butan-2-one 3k. Chromatography on silica gel of the crude product using light petroleumndash;ethyl acetate (8 1) as eluent gave the title compound as an oil (48) (Found M1 236.1233. C14H20OS requires M 236.1235); nmax(CCl4)/cm21 1735 (COCH3); dH(CDCl3) 0.85 (3H t J 8.7 CH3) 1.25 (2H m CH2) 1.80 (2H m CH2) 2.00 (3H s COCH3) 2.48 (4H m 2CH2) 3.75 (1H dd J 7.7 CH) and 7.15ndash;7.3 (5H m ArH); dC(CDCl3) 13.72 (CH3) 20.78 24.86 (CH2) 29.91 (CH3CO) 38.50 43.38 (CH2) 50.01 (CHPh) 127.05 127.77 128.44 (ArCH) 142.74 (ArC) and 206.93 (CO); m/z (EI) 236 (M1) 193 (M1 2 COCH3) 165 (M1 2 CH2CH2- COMe) 133 (M1 2 SCH2CH2CO2Me) and 91 (C7H7).Methyl 3-(19-phenylbutylsulfanyl)propionate 3l. Chromatography on silica gel of the crude product using light petroleumndash;dichloromethane (1 1) as eluent gave the title compound as an oil (49) (Found M1 252.1186. C14H20O2S requires M 252.1184); nmax(CCl4)/cm21 1745 (CO2Me); dH(CDCl3) 0.85 (3H t J 7.5 CH3) 1.3 (2H m CH2) 1.8 (2H m CH2) 2.5 (4H m 2CH2) 3.6 (3H s OCH3) 3.8 (1H dd J 10.7 CH) and 7.25 (5H m ArH); dC(CDCl3) 13.68 (CH3) 20.71 25.84 34.32 38.50 (CH2) 49.58 (CHPh) 51.60 (CH3) 127.02 127.74 128.40 (ArCH) 142.52 (ArC) and 172.28 (CO); m/z (EI) 252 (M1) 209 (M1 2 CH2CH2CH3) 165 (M1 2 CH2CH2CO2Me) 133 (M1 2 SCH2CH2CO2Me) and 91 (C7H7).Ethyl 3-(19-phenyl-39-methylbutylsulfanyl)propionate 3m. Chromatography on silica gel of the crude product using light petroleumndash;ethyl acetate (10 1) as eluent gave the title compound as an oil (60) (Found M1 280.1495. C16H24O2S requires M 280.1497); nmax(CCl4)/cm21 1740 (CO2Et); dH(CDCl3) 0.85 (6H dd J 4.0 2CH3) 1.20 (3H t J 7.2 CH3) 1.5 (1H m CH) 1.7 (2H m CH2) 2.30ndash;2.55 (4H m 2CH2) 3.85 (1H dd J 7.1 CHPh) 4.1 (2H q J 7.2 OCH2) and 7.2ndash; 7.35 (5H m ArCH); dC(CDCl3) 14.12 21.94 22.71 (CH3) 25.62 (CH) 25.80 34.50 45.32 (CH2) 47.64 (CHPh) 60.48 (CH2O) 127.02 127.73 128.43 (ArCH) 142.60 (ArC) and 172.10 (CO); m/z (EI) 280 (M1) 223 M1 2 CH2CH(CH3)2 179 (M1 2 CH2CH2CO2Et) 147 (M1 2 SCH2CH2CO2Et) and 91 (C7H7). Methyl 3-(19-phenyl-39-methybutylsulfanyl)propionate 3n.Chromatography on silica gel of the crude product using light petroleumndash;diethyl ether (20 1) as eluent gave the title compound as an oil (48) (Found M1 266.1348. C15H22O2S requires M 266.13405); nmax(CCl4)/cm21 1745 (CO2Me); dH(CDCl3) 0.90 (6H dd J 7.1 2CH3) 1.50 (1H m CH) 1.70 (2H m CH2) 2.42 (2H m CH2) 2.55 (2H m CH2) 3.65 (3H s OMe) 3.90 (1H dd J 9.5 CHPh) and 7.2ndash;7.35 (5H m ArCH); dC(CDCl3) 21.98 22.74 (CH3) 25.70 (CH) 25.84 (CH2) 34.37 (CH2S) 45.40 (CH2CO) 47.81 (CHPh) 51.68 (CH3O) 127.08 127.78 128.48 (ArCH) 142.64 (ArC) and 172.36 (CO); m/z (EI) 266 (M1) 209 M1 2 CH2CH(CH3)2 179 (M1 2 CH2CH2CO2Me) 147 (M1 2 SCH2CH2CO2Me) and 91 (C7H7). Desilylation of product 1b in the presence of methyl acrylate To a solution of the ester 1b (80 mg 0.25 mmol) in moist THF (3 cm3) were added a solution of tetrabutylammonium fluoride (TBAF) in THF (1.0 M; 0.32 cm3 0.32 mmol) and methyl acrylate (0.022 cm3 21.5 mg 0.25 mmol).The reaction mixture was stirred at reflux temperature for 4 h after which it was diluted with saturated aqueous ammonium chloride and extracted with diethyl ether. The extract was washed several times with water and then dried and concentrated under reduced pressure. Chromatography on silica gel of the crude reaction mixture using light petroleumndash;diethyl ether (30 1) as eluent gave as the higher RF fraction a product arising from the SiMe2Ph moiety as the second RF fraction product 3b (16.6 mg 0.0625 mmol 25) and as the lower RF fraction product 3l (15.7 mg 0.0625 mmol 25). Both products were compared with authentic samples.Desilylation of an equimolar mixture of 1b and 1n To a solution of the ester 1b (80 mg 0.25 mmol) and the ester 1n (80 mg 0.25 mmol) in moist THF (6 cm3) was added a solution of tetrabutylammonium fluoride (TBAF) in THF (1.0 M; 0.64 cm3 0.64 mmol). The reaction mixture was stirred at reflux temperature for 5 h after which it was diluted with saturated aqueous ammonium chloride and extracted with diethyl ether. The extract was washed several times with water and then dried and concentrated under reduced pressure. Chromatography on silica gel of the crude reaction mixture using light petroleumndash;benzene (2 1) as eluent gave as the higher RF fraction a product arising from the SiMe2Ph moiety and as the lower RF fraction a mixture of four products (80 overall yield) in a ratio of 3b 3l 3n :3m = 37 9.5 37 16.The four products were compared with authentic samples and the ratio of the products was estimated by 1H NMR spectrometry. Desilylation of (Z)-1-dimethyl(phenyl)silylbut-1-enethiol 4a 1 A solution of tetrabutylammonium fluoride (TBAF) in THF (1.0 M; 1.2 cm3 1.2 mmol) was added to a solution of (Z)-1- dimethyl(phenyl)silylbut-1-enethiol 4a (0.2 g 0.9 mmol) in moist THF (6 cm3). The reaction mixture was stirred at reflux temperature for 2 h after which it was diluted with saturated aqueous ammonium chloride and extracted with diethyl ether. The organic layer was washed several times with water and then dried and concentrated under reduced pressure. Chromatography on silica gel of the crude reaction mixture using light petroleum as eluent gave as the higher RF fraction a product arising from the SiMe2Ph moiety.The lower RF fraction was a mixture of products 12 and 13 as evidenced by the 1H NMR and GCndash;MS spectra. All the attempts at further separation of products 12 and 13 failed. For this reason we prepared product 12 pure by desilylating 4a (0.170 g 0.77 mmol) in moist acetonitrile (3 cm3) in the presence of solid CsF (0.23 g 1.5 mmol). The reaction mixture was stirred at reflux temperature for 4 h after which it was quenched with water and extracted with diethyl ether. The organic layer was washed several times with water and then dried and concentrated under reduced pressure. Chromatography on silica gel of the crude reaction mixture using light petroleum as eluent gave as the higher RF fraction a product arising from the SiMe2Ph moiety and as the lower RF fraction product 12 (55) as an oil (Found M1 330.1472.C20H26S2 requires M 330.1476); nmax(CCl4)/cm21; dH(CDCl3) 0.85 (3H t J 7.0 CH3) 1.15 (2H m CH2) 1.8 (2H m CH2) 3.30 (1H dd J 8.0 CHPh) and 7.2ndash;7.4 (5H m ArCH); dC(CDCl3) 13.72 (CH3) 20.78 36.73 (CH2) 55.07 (CHPh) 127.23 128.27 128.34 (ArCH) and 139.18 (ArC); m/z (EI) 330 (M1) 198 (M1 2 C3H7CPh) 133 (CH3CH2CH2- CHPh) 117 (133 2 CH2) and 91 (C7H7). The structure of product 13 was tentatively assigned on the basis of its mass spectrum m/z (GC-MS EI) 222 (M1) 179 (M1 2 C3H7) 133 (CH3CH2CH2CHPh) 117 (133-CH2) and 91 (C7H7). Acknowledgements This work was supported by the Ministero dellrsquo;Universitagrave; e della Ricerca Scientifica e Tecnologica (MURST Italy). The authors thank Prof. P. Knochel (University of Marburg) for useful discussions.References 1 B. F. Bonini M. Comes Franchini M. Fochi G. Mazzanti F. Peri and A. Ricci J. Chem. Soc. Perkin Trans. 1 1996 2803. 2 (a) B. Th. Grouml;bel and D. Seebach Chem. Ber. 1977 110 852; (b) R. D. Miller and R. Hassig Tetrahedron Lett. 1984 25 5351; (c) B. Harirchian and P. Magnus J. Chem. Soc. Chem. Commun. 1977 522; (d) F. Cooke R. Moerck J. Schwindeman and P. Magnus J. Org. Chem. 1980 45 1046. J. Chem. Soc. Perkin Trans. 1 1997 3217 3 (a) K. S. Kyler and D. S. Watt J. Org. Chem. 1981 46 5182; (b) T. Mandai M. Kohama H. Sato M. Kawada and J. Tsuji Tetrahedron 1990 46 4553. 4 (a) P. Magnus and D. A. Quagliato Organometallics 1982 1 1240; (b) P. Magnus and D. A. Quagliato J. Org. Chem. 1985 50 1621. 5 B. F. Bonini M. Comes Franchini M.Fochi G. Mazzanti and A. Ricci Tetrahedron 1996 52 4803. 6 B. F. Bonini M. Comes Franchini M. Fochi G. Mazzanti and A. Ricci Tetrahedron 1997 53 7897. 7 For the conversion of acetals into thioacetals see Corey and Seebach Angew. Chem. Int. Ed. Engl. 1965 4 1075. 8 J. March Advanced Organic Chemistry J. Wiley and Sons New York 1985 III edn. p. 687. 9 D. P. Cox J. Terpinski and W. Lawrynwicz J. Org. Chem. 1989 49 3216. 10 B. F. Bonini G. Mazzanti P. Zani and G. Maccagnani J. Chem. Soc. Perkin Trans 1 1989 2083. 11 I. Fleming T. W. Newton and F. Roessler J. Chem. Soc. Perkin Trans 1 1981 2527. Paper 7/03662K Received 27th May 1997 Accepted 21st July 1997 J. Chem. Soc. Perkin Trans. 1 1997 3211 Desilylation of (Z)-middot;-dimethylphenylsilyl vinyl sulfides with fluoride ion 1 revised mechanism for phenyl group migration in substrates containing an electron-withdrawing group lsquor; to the sulfur Bianca Flavia Bonini,* Mauro Comes Franchini Mariafrancesca Fochi Germana Mazzanti and Alfredo Ricci Dipartimento di Chimica Organica lsquo;A.Manginirsquo; dellrsquo;Universitagrave; di Bologna Viale Risorgimento n. 4 I-40136 Bologna Italy A detailed investigation of the desilylation of (Z)-a-dimethylphenylsilyl vinyl sulfides with fluoride ion has shown that in substrates containing an electron-withdrawing group b to the sulfur phenyl group migration occurs as a result of a fluoride ion catalyzed retro-Michael reaction. Introduction a-Silyl vinyl sulfides 2andash;d,3a,b are intriguing species since they can react both as vinylsilanes and vinyl sulfides.4a,b Recently we synthesized a number of these compounds and investigated their chemistry.1,5,6 During our study on the protiodesilylation of the a-silyl vinyl sulfides 1 with fluoride ion,1 we found that desilylation of (Z)-a-trimethylsilyl vinyl sulfides 1 (R3 = Me) can be readily achieved to afford stereoselectively the corresponding (Z)-vinyl sulfides.In contrast desilylation of compounds 1 (R3 = Ph) gave unexpected results. In fact while treatment of 1a with moist TBAF in boiling THF gave the desilylated (Z)-a-vinyl sulfide 2a the reaction of the homologous compound 1b under the same conditions gave 3a arising from migration of the phenyl group from the silicon to the adjacent carbon atom (Scheme 1). The different behaviour of 1a and 1b upon desilylation indicates that structural features are crucial for phenyl group migration.According to these preliminary results 1 we attributed the phenyl group migration in 1b to greater nucleophilicity of its sulfur atom compared with that of 1a. Thus compound 1b can be easily protonated in the b position and this initiates the phenyl group migration.1 In this paper we report the full details of the desilylation of (Z)-a-dimethylphenyl silyl vinyl sulfides and provide a new rationale for the phenyl group migration. Results and discussion In order to investigate in detail the desilylation of (Z)-adimethylphenylsilyl vinyl sulfides 1 we synthesized a variety of compounds 1 R3 = Ph R1 = Et Pri n = 1 2 3 and R2 = an electron-withdrawing group (EWG) an electron-donating R1 S SiMe2R3 R2 1 ( )n Scheme 1 S CO2Et Ph SiMe2Ph S CO2 Et H S CO2 Et H n = 2 TBAF THF H2O heat 2a 3a n = 1 1a n = 2 1b n = 1 ( )n group (EDG) or an alkyl group.Products 1 were prepared from the corresponding (Z)-a-dimethylphenylsilyl enethiols 4 1 with two different procedures illustrated in Scheme 2 and Table 1. The full details of the synthesis of (Z)-1-dimethyl(phenyl)- silylbut-1-enethiol 4a have been previously reported; 1 the (Z)- 1-dimethyl(phenyl)silyl-3-methylbut-1-enethiol 4b has been prepared by a similar procedure to that used to prepare 4a (see Experimental section). According to path A products 1andash;i were obtained in a stereoselective manner by reaction of 4 with halides R2(CH2)nX in acetone in the presence of dry K2CO3 at room temperature; Scheme 2 R X SH R1 SiMe2Ph S R1 SiMe2Ph R2 R S R1 SiMe2Ph R2 1andash;i R2 = EWG EDG alkyl Acetone K2CO3 THF DBU 1b 1jndash;n R2 = EWG 4a R1 = Et 4b R1 = Pri path A path B ( )n ( )n Table 1 Synthesis of (Z)-a-dimethylphenylsilyl vinyl sulfides 1 (paths A and path B) 1 a b b c d e e f g h i j k l m n R1 Et Et Et Et Et Et Et Et Et Et Et Et Et Et Pri Pri n 1 2 1 1 1 1 2 1 2 3 R2 CO2Et CO2Et CO2Et CN COMe OEt OEt OEt H H H CN COMe CO2Me CO2Et CO2Me X I Cl Cl Cl Cl Cl Cl I Br Br Path A A B A A A A A A A B B B B B Yield () b 80 a 80 a 80 89 69 20 c 90d 26 95 a 90 80 91 66 92 84 86 a Ref 1.b The yields were determined after chromatography. c Beside 1e product 5 was obtained in 25 yield. d Prepared using triethylamine as the base (see text). 3212 J. Chem. Soc. Perkin Trans. 1 1997 good yields were generally obtained except in the case of halides containing an ethoxy group in the a- or in a b-position.During the synthesis of 1e (20 yield) the thioacetal 5 was formed in 25 yield arising probably by an acid-catalyzed reaction 7 (HCl) of the O,S-thioacetal 1e with the starting enethiols 4a (Scheme 3). The structure of product 5 was established on the basis of analytical and spectral data (see Experimental section). In an effort to improve the yield of products 1e and 1f we allowed 4a to react with chloromethyl ethyl ether in diethyl ether in the presence of an equimolar amount of triethylamine to give product 1e (90) yield; attempted reaction of 4a with chloroethyl ethyl ether gave under similar reaction conditions recovery of 4a. By path B (Scheme 2) the enethiols 4 gave a base- (DBU) catalyzed Michael type addition 8 with olefins bearing an electron-withdrawing group to afford (Z)-a-dimethylphenylsilyl vinyl sulfides 1b 1jndash;n in very good yields (Table 1).Compounds 1andash;n were desilylated by reaction with TBAF in moist THF at reflux. Normal protiodesilylation occurs with the following substrates containing an electron-withdrawing group a to the sulfur 1a c d; substrates containing an electrondonating group a or b to the sulfur 1e f; and substrates with an alkyl group bonded to the sulfur 1gndash;i (Scheme 4). The yield of products 2 were generally good (Table 2) except for 2g where the product was volatile. Desilylation of 1d gave beside product 2d a cyclic compound 6 (18) arising from an intramolecular cyclization of the intermediate vinyl anion formed during the desilylation on the carbonyl group. In contrast the derivatives containing an electron-withdrawing group b to the sulfur 1b jndash;n gave upon treatment with moist TBAF in boiling THF products 3 the result of phenyl group migration (Scheme 5 Table 3).These results clearly show the inadequacy of our previous Scheme 3 Et S Si OEt Et Si S S Si Et 5 1e + 4a Si = SiMe2Ph Scheme 4 S SiMe2Ph R2 S H H R2 2a cndash;i THF H2O heat TBAF 1a c-i ( )n ( )n S Et Me HO 6 Table 2 Protiodesilylation of (Z)-a-dimethylphenylsilyl vinyl sulfides 1 1 a c d e f g h i n 1 1 1 1 2 1 2 3 R2 CO2Et CN COMe OEt OEt H H H 2 a c d e f g h i Yield () 80a 72 82 b 95 98 46 c 98 75 a Compared with an authentic sample (ref. 1). b Beside product 2d a cyclic product 6 was obtained in 18 yield (see Experimental section). c Ref. 1. mechanism.1 In fact substrates containing an electrondonating group in a position a or b to the sulfur which would increase the availability of electron density on the sulfur did not favour phenyl group migration.A plausible mechanistic interpretation of the phenyl group migration is outlined in Scheme 6. Substrates 1 containing a hydrogen atom a to an electronwithdrawing group can be easily deprotonated by a base such as TBAF. In fact TBAF behaves not only as a potent source of nucleophilic fluoride but also as a potent base.9 If the produced carbanion can undergo a retro-Michael reaction as in the case of the intermediate 7 extrusion of the olefins 8 takes place to give the enethiolate 9. The silicon may or may not have a fluoride ion coordinated to it in these stages. The phenyl group in the thione 10 can then migrate from the silicon to the adjacent carbon to give 11; this by a Michael addition to the olefin 8 affords after desilylation product 3.To test this hypothesis we desilylated 1b in the presence of 1 equiv. of methyl acrylate to obtain compounds 3b and 3l; these arose from the thiolate 11a (R1 = Et) as a result of Michael addition with ethyl acrylate and methyl acrylate followed by desilylation (Scheme 7). A further proof of the mechanism depicted in Scheme 6 was obtained by desilylation of equimolar amounts of 1b and 1n under the usual experimental conditions when four crossed products 3b 3l 3n and 3m were obtained in the yields reported in Scheme 8. These yields were calculated from the 1H NMR spectrum of the crude mixture by comparison with the 1H NMR spectra of authentic samples (see Scheme 5 R1 S SiMe2Ph R2 R1 S R2 Ph 3b jndash;n 1b jndash;n THF H2O heat TBAF R2 = EWG Scheme 6 R1 Ph S EWG EWG R1 Sndash; SiMe2F Ph S Si R1 Me Ph Me S SiMe2Ph R1 R1 Sndash; SiMe2Ph EWG R1 S SiMe2Ph EWG R1 S SiMe2Ph EWG Fndash; Fndash; Fndash; + SiMe2F2 1b jndash;n 7 9 10 11 3 ndash; 8 8 H+/Fndash; Table 3 1 b j k l m n R1 Et Et Et Et Pri Pri R2 CO2Et CN COMe CO2Me CO2Et CO2Me 3 b j k l m n Yield () 48 a 48 48 49 60 48 a Ref.1. J. Chem. Soc. Perkin Trans. 1 1997 3213 Experimental section). The four products arise from the reaction of the two intermediates 11a (R1 = Et in Scheme 6) and 11b (R1 = Pri in Scheme 6) with ethyl acrylate and methyl acrylate which are formed in the retro-Michael reaction stage. We then examined the desilylation of the enethiol 4a which was expected to give the same intermediate 11a. In fact treatment of 4a with TBAF in boiling THF gave an inseparable mixture of the disulfide 12 arising from dimerization of the thiolate 11a followed by desilylation and of product 13 (Scheme 9).The formation of product 13 is probably the result of alkylation of the thiolate 11a (R1 = Et in Scheme 6) either by the TBAF itself or by other species deriving from its decomposition at higher temperature.10 Product 12 could be obtained pure by performing the desilylation of 4a with CsF in boiling Me3CN (Scheme 9); its structure was established on the basis of satisfactory analytical and spectral results. Conclusion In summary we have examined the desilylation of (Z)-adimethylphenylsilyl vinyl sulfides 1 with fluoride ion and rationalized the migration of the phenyl group from the silicon to the adjacent carbon atom in substrates containing an electron-withdrawing group b to the sulfur.Substrates containing an electron-withdrawing group a to the sulfur or an electron-donating group a or b to the sulfur or an alkyl chain bonded to the sulfur gave compounds arising from normal protiodesilylation. Experimental Bps and mps are uncorrected. 1H NMR and 13C NMR spectra Scheme 7 S SiMe2Ph CO2Et Ph S CO2Me CO2Me Ph S CO2Et 1b 3l 3b THF H2O heat TBAF 25 25 Scheme 8 S SiMe2Ph CO2Et Ph S CO2Me Ph S CO2Et S SiMe2Ph CO2Me Ph S CO2Et Ph S CO2Me 13 30 7.5 30 Yield () THF H2O heat TBAF + 1b 1n 3b 3l 3n 3m Scheme 9 Ph S Ph S Ph S Bu SH SiMe2Ph THF H2O heat TBAF CsF + 12 13 12 4a MeCN H2O heat )2 )2 were recorded with Varian Gemini 200 or 300 MHz spectrometers as solutions in CDCl3 chemical shifts (d) are given in ppm relative to tetramethylsilane TMS.J Values are given in Hz. 13C NMR spectra assignments were made by DEPT experiments. Mass spectra were obtained using a VG 7070-E (EI 70 eV) spectrometer. IR spectra were recorded on a Perkin- Elmer model 257 grating spectrometer. Reactions were conducted in oven-dried (120 8C) glassware under a positive argon atmosphere. Transfer of anhydrous solvents or mixtures was accomplished with oven-dried syringes. THF was distilled from sodium benzophenone just prior to use and stored under argon. Et2O was distilled from phosphorus pentoxide. The reactions were monitored by TLC performed on silica gel plates (Baker- flex IB2-F). Column chromatography was performed with Merck silica gel 60 (70ndash;230 mesh) and preparative thick layer chromatography was carried out on glass plates using a 1-mm layer of Merck silica gel 60 Pf254 or aluminium oxide F254.Light petroleum refers to the fraction with bp 40ndash;60 8C. Because of the small scale used in the preparations new compounds which were oily products have been characterized by accurate mass measurements. All chemicals were used as obtained or purified by distillation as needed. Sodium hydrogen carbonate 99 was purchased from Carlo Erba Reagenti; hydrogen chloride was purchased from Praxair (Belgium). 3-Methylbutanoyl(dimethyl)phenylsilane 3-Methylbutanoyl chloride (1.20 g 1.22 cm3 10.0 mmol) in anhydrous THF (3 cm3) was added slowly to lithium bis- dimethyl(phenyl)silylcuprate 11 (10.0 mmol) at 278 8C under argon. The mixture was stirred at 278 8C for 1 h and then allowed to warm to 0 8C at which temperature it was stirred for 1 h.The mixture was then treated with saturated aqueous ammonium chloride to quench the reaction and extracted with diethyl ether. The extract was dried and concentrated under reduced pressure. Chromatography of the residue on silica gel column light petroleumndash;diethyl ether (10 1) as eluent gave as the higher RF fraction a product arising from the silylcuprate and as the lower RF fraction the acylsilane (1.6 g 73) as a yellow oil (Found M1 220.1379. C13H20OSi requires M 220.1283); nmax(CCl4)/cm21 1650 (CO) 1430 1110 (SiPh) and 1250 (SiMe2); dH(CDCl3) 0.45 (6H s SiMe2) 0.85 (6H d J 6.6 CH3) 2.2 (2H m CH) 2.50 (2H d J 6.7 CH2CO) 7.45ndash;7.50 (3H m ArH) and 7.58ndash;7.62 (2H m ArH); dC(CDCl3) 24.86 (SiMe2) 22.54 (2CH3) 22.86 (CH) 57.58 (CH2CO) 128.01 129.71 133.85 (ArCH) 139.15 (ArC) and 246.51 (CO); m/z (EI) 220 (M1) 163 M1 2 (CH3)2CHCH2 and 135 (SiMe2Ph).(Z)-1-Dimethyl(phenyl)silyl-3-methylbut-1-enethiol 4b Hydrogen chloride and hydrogen sulfide were bubbled into a solution of 3-methylbutanoyl(dimethyl)phenylsilane (0.35 g 1.59 mmol) in diethyl ether (70 cm3) at 220 8C until the starting ketone had disappeared TLC with light petroleumndash;diethyl ether (10 1) as eluent. After the mixture has been allowed to warm to room temperature it was treated with solid sodium hydrogen carbonate until evolution of carbon dioxide ceased; it was then left overnight. Filtration and concentration of the mixture under reduced pressure gave the pure (Z)-enethiol 4b as an oil (0.35 g 1.48 mmol 93) (Found M1 236.1059. C13H20SSi requires M 236.1055); nmax(CCl4)/cm21 2540 (SH) 1430 1110 (SiPh) 1250 and 890 (SiMe2); dH(CDCl3) 0.45 (6H s SiMe2) 1.05 (6H d J 10.0 2CH3) 2.40 (1H s SH) 2.65ndash;2.85 (1H m CH) 5.80 (1H d J 9.0 vinylic H) 7.40 (3H m ArH) and 7.70 (2H m ArH); dC(CDCl3) 23.16 (SiMe2) 21.82 (2CH3) 29.89 (CH) 125.24 (C) 127.94 129.44 134.18 (ArCH) 136.57 (C) and 147.52 (vinylic CH); m/z (EI) 236 (M1) 221 (M1 2 CH3) 187 (M1 2 H2S) 158 (M1 2 C6H6) 143 (M1 2 C7H9) and 135 (SiMe2Ph).Synthesis of (Z)-middot;-silyl vinyl sulfides 1 general method (Path A). To a solution of (Z)-a-silyl enethiol 4 (1.0 mmol) in acetone (4 cm3) solid oven-dried K2CO3 (1.3 mmol) and the halide (1.1 3214 J. Chem. Soc. Perkin Trans. 1 1997 mmol) were added. The mixture was stirred at room temperature until the starting enethiol had disappeared. The mixture was then diluted with water and extracted with diethyl ether.The extract was dried and concentrated to give the title product. In some cases the product was purified by chromatography on silica light petroleumndash;diethyl ether (10 1) as eluent. (Z)-{1-Dimethyl(phenyl)silylbut-1-enylsulfanyl}acetonitrile 1c. Yield 89 as an oil (Found M1 261.1012. C14H19NSSi requires M 261.1007); nmax(CCl4)/cm21 2242 (CN) 1426 (SiPh) 1248 (SiMe2) and 1109 (SiPh); dH(CDCl3) 0.50 (6H s SiMe2) 1.05 (3H t J 7.5 CH3) 2.55 (2H m CH2) 2.85 (2H s CH2CN) 6.65 (1H t J 6.9 vinylic H) 7.38 (3H m ArH) and 7.60 (2H m ArH); dC(CDCl3) 22.90 (SiMe2) 13.50 (CH3) 19.55 24.45 (CH2) 116.65 (CN) 128.01 129.58 133.93 (ArCH) 135.10 (ArC) 136.90 (vinylic C) and 158.73 (vinylic CH); m/z (EI) 261 (M1) 260 (M1 2 1) 246 (M1 2 CH3) 221 (M1 2 CH2CN) and 135 (SiMe2Ph).(Z)-{1-Dimethyl(phenyl)silylbut-1-enylsulfanyl}propan-2- one 1d. Chromatography on silica gel of the crude reaction mixture using light petroleumndash;diethyl ether (8 1) as eluent gave the title product as oil (70) (Found M1 278.1158. C15H22- OSSi requires M 278.1161); nmax(CCl4)/cm21 1720 (CO) 1440 (SiPh) 1240 (SiMe2) and 1110 (SiPh); dH(CDCl3) 0.45 (6H s SiMe2) 1.00 (3H t J 7.5 CH3) 2.00 (3H s COCH3) 2.40 (2H m CH2) 3.10 (2H s CH2CO) 6.35 (1H t J 6.8 vinylic H) 7.35 (3H m ArH) and 7.65 (2H m ArH); dC(CDCl3) 22.49 (SiMe2) 13.41 (CH3) 24.32 (CH2) 27.96 (CH3) 44.57 (SCH2) 127.82 129.25 133.98 (ArCH) 131.15 (ArC) 137.62 (vinylic C) 154.73 (vinylic CH) and 203.18 (CO); m/z (EI) 278 (M1) 263 (M1 2 CH3) and 135 (SiMe2Ph). (Z)-1-Dimethyl(phenyl)silyl-1-ethoxymethylsulfanylbut-1- ene 1e.Chromatography on alumina of the crude product using light petroleumndash;diethyl ether (20 1) as eluent gave as the higher RF fraction product 5 as an oil (25) (Found M1 456.1792. C25H36S2Si2 requires M 456.1797); nmax(CCl4)/cm21 1430 1110 (SiPh) and 1250 (SiMe2); dH(CDCl3) 0.40 (12H s 2 SiMe2) 1.00 (6H t J 7.5 2 CH3) 2.40 (4H m 2CH2) 3.4 (2H s SCH2S) 6.30 (2H t J 6.8 2 vinylic H) 7.4 (6H m ArH) and 7.6 (4H m ArH); dC(CDCl3) 22.35 (SiMe2) 13.39 (CH3) 24.37 40.03 (CH2) 127.74 129.10 134.00 (ArCH) 132.72 (ArC) 137.86 (vinylic C) and 153.77 (vinylic CH); m/z (EI) 456 (M1) 221 (CH3CH2CHCSSiMe2Ph) 144 (CH3CH2- CHCSCH2SC) 135 (SiMe2Ph) and 91 (C7H7); as the second RF fraction the title product as an oil (20) (Found M1 280.1315. C15H24OSSi requires M 280.1317); nmax(CCl4)/cm21 1080 1140 (C-O-C) 1430 1110 (SiPh) and 1250 (SiMe2); dH(CDCl3) 0.45 (6H s SiMe2) 1.05 (3H t J 7.6 CH3) 1.15 (3H t J 7.1 CH3) 2.45 (2H m CH2) 3.5 (2H q J 7.1 OCH2) 4.45 (2H s CH2O) 6.35 (1H t J 6.8 vinylic H) 7.35 (3H m ArH) and 7.65 (2H m ArH); dC(CDCl3) 22.55 (SiMe2) 13.39 14.70 (CH3) 24.32 63.99 75.91 (CH2) 127.68 129.06 134.08 (ArCH) 131.89 (ArC) 139.00 (vinylic C) and 152.42 (vinylic CH); m/z (EI) 280 (M1) 221 (M1 2 CH2OC2H5) and 135 (SiMe2Ph).Improved procedure for 1e. To a solution of (Z)-1-dimethyl- (phenyl)silylbut-1-enethiol (0.13 g 0.59 mmol) in anhydrous diethyl ether (3 cm3) chloromethyl ethyl ether (0.065 g 0.07 cm3 0.7 mmol) and triethylamine (0.07 g 0.1 cm3 0.7 mmol) were added. The mixture was stirred at room temperature until the starting enethiol had disappeared TLC light petroleumndash; diethyl ether (20 1) as eluent 10 min..The mixture was then diluted with water and extracted with diethyl ether. The organic layer was dried and concentrated to give the title product as an oil (1.15 g 0.53 mmol 90). Use of an excess of chloromethyl ethyl ether and triethylamine gave the title product in quantitative yield. (Z)-1-Dimethyl(phenyl)silyl-1-(29-ethoxyethylsulfanyl)but-1- ene 1f. Chromatography on silica gel of the crude product using light petroleumndash;diethyl ether (10 1) as eluent gave the title product as an oil (26) (Found M1 294.1478. C16H26OSSi requires M 294.1474); nmax(CCl4)/cm21 1430 1110 (SiPh) 1248 (SiMe2) and 1150ndash;1070 (C-O-C); dH(CDCl3) 0.45 (6H s SiMe2) 1.05 (3H t J 7.6 CH3) 1.15 (3H t J 7.1 CH3) 2.45 (2H m CH2) 2.62 (2H t J 7.2 SCH2) 3.30 (2H t J 7.2 CH2) 3.34 (2H q J 7.1 CH2O) 6.3 (1H t J 6.8 vinylic H) 7.35 (3H m ArH) and 7.45 (2H m ArH); dC(CDCl3) 22.37 (SiMe2) 13.37 15.11 (CH3) 24.22 33.64 66.05 69.66 (CH2) 127.73 129.07 133.95 (ArCH) 132.20 (ArC) 138.60 (vinylic C) and 152.90 (vinylic CH); m/z (EI) 294 (M1) 249 (M1 2 OC2H5) 221 (M1 2 CH2CH2OC2H5) 179 (221 2 C3H6) and 135 (SiMe2Ph).A reaction performed using triethylamine as a base in diethyl ether with the (Z)-enethiol 4a gave recovery of starting material. (Z)-1-Dimethyl(phenyl)silyl-1-ethylsulfanylbut-1-ene 1h. Yield 90 as an oil (Found M1 250.1216. C14H22SSi requires M 250.121 15); nmax(CCl4)/cm21 1430 (SiPh) 1250 (SiMe2) and 1100 (SiPh); dH(CDCl3) 0.50 (6H s SiMe2) 1.03 (3H t J 7.4 CH3) 1.06 (3H t J 7.5 2CH3) 2.45 (4H m 2CH2) 6.30 (1H t J 6.5 vinylic H) 7.38 (3H m ArH) and 7.60 (2H m ArH); dC(CDCl3) 22.30 (SiMe2) 13.39 14.81 (CH3) 24.14 28.42 (CH2) 127.66 128.99 133.93 (ArCH) 132.80 (ArC) 138.05 (vinylic C) and 152.194 (vinylic CH); m/z (EI) 250 (M1) 221 (M1 2 C2H5) and 135 (SiMe2Ph).(Z)-1-Dimethyl(phenyl)silyl-1-propylsulfanylbut-1-ene 1i. Yield 80 as an oil (Found M1 264.1362. C15H24SSi requires M 264.1368); nmax(CCl4)/cm21 1426 (SiPh) 1246 (SiMe2) and 1108 (SiPh); dH(CDCl3) 0.40 (6H s SiMe2) 0.80 (3H t J 7.3 CH3) 1.00 (3H t J 7.6 CH3) 1.40 (2H m CH2) 2.40 (4H m 2CH2) 6.25 (1H t J 6.7 vinylic H) 7.35 (3H m ArH) and 7.65 (2H m ArH); dC(CDCl3) 22.32 (SiMe2) 13.24 13.36 (CH3) 23.22 24.12 36.37 (CH2) 127.61 128.93 133.87 (ArCH) 134.60 (ArC) 138.22 (vinylic C) and 152.03 (vinylic CH); m/z (EI) 264 (M1) 221 (M1 2 C3H7) and 135 (SiMe2- Ph).Synthesis of (Z)-middot;-silyl vinyl sulfides 1 general method (Path B) To a solution of (Z)-a-silyl enethiol (1.0 mmol) in THF (5 cm3) 1,8-diazabicyclo5.4.0undec-7-ene (DBU) (1.1 mmol) and the olefin (1.1 mmol) were added. The mixture was stirred at room temperature until the starting enethiol had disappeared. The mixture was then treated with water and extracted with diethyl ether. The extract was dried and concentrated to give the title product. All the yields were calculated on the crude products. The crude products were used for further reactions without purification. In some cases the products were purified by chromatography on silica light petroleumndash;diethyl ether (10 1) as eluent for the complete characterization analysis. 3-{(Z)-1-Dimethyl(phenyl)silylbut-1-enylsulfanyl}propionitrile 1j.Yield 91 as an oil (Found M1 275.1168. C15H21 NSSi requires M 275.1164); nmax(CCl4)/cm21 2248 (CN) 1426 1109 (SiPh) and 1248 (SiMe2); dH(CDCl3) 0.45 (6H s SiMe2) 1.05 (3H t J 7.6 CH3) 2.15 (2H t J 7.2 CH2) 2.45 (2H m CH2) 2.55 (2H t J 7.2 CH2) 6.45 (1H t J 6.8 vinylic H) 7.35 (3H m ArH) and 7.65 (2H m ArH); dC(CDCl3) 22.68 (SiMe2) 13.38 (CH3) 18.22 24.34 29.36 (CH2) 118.20 (CN) 128.01 129.50 133.81 (ArCH) 131.12 (ArC) 137.65 (vinylic C) and 155.03 (vinylic CH); m/z (EI) 275 (M1) 260 (M1 2 CH3) 246 (M1 2 C2H5) 221 (M1 2 CH2CH2CN) and 135 (SiMe2Ph). 4-{(Z)-1-Dimethyl(phenyl)silylbut-1-enylsulfanyl}butan-2- one 1k. Chromatography on silica gel of the crude product using light petroleumndash;diethyl ether (10 1) as eluent gave the title product as an oil (66) (Found M1 292.1319.C16H24OSSi requires M 292.1317); nmax(CCl4)/cm21 1725 (COMe) 1430 1110 (SiPh) and 1250 (SiMe2); dH(CDCl3) 0.45 (6H s SiMe2) 1.00 (3H t J 7.6 CH3) 1.95 (3H s COCH3) 2.3ndash;2.4 (4H m 2CH2) 2.6 (2H t J 7.2 CH2) 6.3 (1H t J 6.7 vinylic H) 7.35 (3H m ArH) and 7.65 (2H m ArH); dC(CDCl3) 22.55 (SiMe2) 13.38 (CH3) 24.21 27.98 (CH2) 29.68 (COCH3) 43.68 (CH2) 127.74 129.11 133.90 (ArCH) 132.46 (ArC), J. Chem. Soc. Perkin Trans. 1 1997 3215 138.02 (vinylic C) 152.85 (vinylic CH) and 206.74 (CO); m/z (EI) 292 (M1) 277 (M1 2 CH3) 221 (M1 2 CH2CH2COMe) 189 (M1 2 SCH2CH2COMe) and 135 (SiMe2Ph). Ethyl 3-{(Z)-1-dimethyl(phenyl)silylbut-1-enylsulfanyl}propionate 1b. Yield 80 the product was compared with an authentic sample.1 Methyl 3-{(Z)-1-dimethyl(phenyl)silylbut-1-enylsulfanyl}- propionate 1l.Yield 92 as an oil (Found M1 308.1264. C16H24O2SSi requires M 308.1266); nmax(CCl4)/cm21 1745 (CO2Me) 1435 1115 (SiPh) and 1250 (SiMe2); dH(CDCl3) 0.40 (6H s SiMe2) 0.95 (3H t J 7.6 CH3) 2.29 (2H t J 7.1 CH2) 2.38 (2H m CH2) 2.62 (2H t J 7.7 CH2) 3.60 (3H s OCH3) 6.30 (1H t J 6.6 vinylic H) 7.30 (3H m ArH) and 7.55 (2H m ArH); dC(CDCl3) 22.48 (SiMe2) 13.37 (CH3) 24.23 29.03 34.50 (CH2) 51.60 (OCH3) 127.76 129.11 133.90 (ArCH) 131.93 (ArC) 137.92 (vinylic C) 153.67 (vinylic CH) and 172.28 (CO); m/z (EI) 308 (M1) 222 (M1 2 CHCH2CO2Me) and 135 (SiMe2Ph). Ethyl 3-{(Z)-1-dimethyl(phenyl)silyl-3-methylbut-1-enylsulfanyl} propionate 1m. Yield 84 as an oil (Found M1 336.1573. C18H28O2SSi requires M 336.1579); nmax(CCl4)/cm21 1740 (CO2Et) 1435 1115 (SiPh) and 1250 (SiMe2); dH(CDCl3) 0.45 (6H s SiMe2) 1.00 (6H d J 6.7 2CH3) 1.20 (3H t J 7.3 CH3) 2.30 (2H t J 7.5 CH2) 2.60 (2H t J 7.5 CH2) 3.15 (1H m CH) 4.09 (2H q J 7.3 OCH2) 6.15 (1H d J 8.9 vinylic H) 7.35 (3H m ArH) and 7.55 (2H m ArH); dC(CDCl3) 22.27 (SiMe2) 14.16 (CH3) 22.22 (2CH3) 29.11 (CH2) 29.60 (CH) 34.81 (CH2) 60.41 (OCH2) 127.76 129.08 133.84 (ArCH) 129.72 (ArC) 138.13 (vinylic C) 159.15 (vinylic CH) and 171.78 (CO); m/z (EI) 336 (M1) 235 (M1 2 CH2CH2- CO2Et) and 135 (SiMe2Ph).Methyl 3-{(Z)-1-dimethyl(phenyl)silyl-3-methylbut-1-enylsulfanyl} propionate 1n. Yield 86 as an oil (Found M1 322.1426. C17H26O2SSi requires M 322.1423); nmax(CCl4)/cm21 1750 (CO2Me) 1435 1115 (SiPh) and 1250 (SiMe2); dH(CDCl3) 0.45 (6H s SiMe2) 1.00 (6H d J 6.6 2CH3) 2.30 (2H t J 7.4 CH2) 2.65 (2H t J 7.4 CH2) 3.20 (1H m CH) 3.62 (3H s OCH3) 6.20 (1H d J 9.1 vinylic H) 7.35 (3H m ArH) and 7.65 (2H m ArH); dC(CDCl3) 22.30 (SiMe2) 22.18 (2 CH3) 29.04 (CH2) 29.58 (CH) 34.55 (CH2) 51.49 (OCH3) 127.74 129.05 133.81 (ArCH) 129.85 (ArC) 138.06 (vinylic C) 159.13 (vinylic CH) and 172.13 (CO); m/z (EI) 322 (M1) 235 (M1 2 CH2CH2CO2Me) and 135 (SiMe2Ph).General procedure for the desilylation of (Z)-middot;-dimethylphenylsilyl vinyl sulfides 1 A solution of tetrabutylammonium fluoride (TBAF) in THF (1.0 M; 1.1 mmol) was added to a solution of (Z)-a-silyl vinyl sulfides (1 mmol) in moist THF (6 cm3). The reaction mixture was stirred at reflux temperature until the starting a-silyl vinyl sulfide had disappeared after which it was diluted with saturated aqueous ammonium chloride to quench the reaction and extracted with diethyl ether.The extract was washed several times with water and then dried and concentrated under reduced pressure. The crude product was purified by preparative thick layer chromatography to give as the higher RF fraction a product arising from the SiMe2Ph moiety and as the lower RF fraction the desilylated product. (Z)-But-1-enylsulfanylacetonitrile 2c. Chromatography on silica gel of the crude product using light petroleumndash;diethyl ether (10 1) as eluent gave the title product as an oil (72) (Found M1 127.0459. C6H9NS requires M 127.0456); nmax(CCl4)/cm21 2260 (CN); dH(CDCl3) 1.05 (3H t J 7.5 CH3) 2.25 (2H m CH2) 3.4 (2H s CH2CN) 5.94 (1H dt J1 9.2 J2 6.9 vinylic H) and 6.02 (1H dt J1 9.2 J2 1.1 vinylic H); dC(CDCl3) 13.44 (CH3) 18.92 22.65 (CH2) 117.15 (CN) and 119.30 and 137.88 (vinylic CH); m/z (EI) 127 (M1) and 112 (M1 2 CH3).(Z)-But-1-enylsulfanylpropan-2-one 2d. A mixture of two products was obtained as shown by the 1H NMR spectrum of the crude reaction mixture the title product (dH 5.60ndash;5.73 and 5.79ndash;5.85) (82) and a cyclic product 6 (dH 5.60) (18). Since the cyclic product 6 was found to be rather unstable on silica it was characterized in the crude mixture nmax(CCl4/cm21 3500 br (OH); dH(CDCl3) 0.95 (3H t J 7.6 CH3) 1.50 (3H s CH3) 1.85 (2H m CH2) 3.05ndash;3.30 (2H dd J 10 CH2) and 5.60 (1H t J 7.2 vinylic H); dC(CDCl3) 13.44 (CH3) 21.13 (CH2) 26.81 (CH3) 29.80 (C) 41.65 (CH2S) 118.57 (vinylic CH) and 144.80 (vinylic C); m/z (EI) 144 (M1) 129 (M1 2 CH3) and 101 (M1 2 COCH3). Chromatography on silica gel of the crude product using light petroleumndash;dichloromethane (2 1) as eluent gave as the higher RF fraction a product arising from the SiMe2Ph moiety and as the second RF fraction the desilylated product as an oil (65) (Found M1 144.0612.C7H12OS requires M 144.0609); nmax(CCl4)/cm21 1715 (COCH3); dH(CDCl3) 1.05 (3H t J 7.6 CH3) 2.15 (2H m CH2) 2.3 (3H s COCH3) 3.35 (2H s SCH2CO) 5.60ndash;5.73 (1H dt J1 9.3 J2 6.9 vinylic H) and 5.79ndash;5.85 (1H dt J1 9.3 J2 1.2 vinylic H); dC(CDCl3) 13.47 (CH3) 22.46 (CH2) 27.66 (CH3) 43.46 (SCH2CO) 121.68 and 133.87 (vinylic CH) and 203.42 (CO); m/z (EI) 144 (M1) 101 (M1 2 COMe) 87 (M1 2 CH2COMe) 55 (M1 2 SCH2COMe) and 43 (COMe). (Z)-1-Ethoxymethylsulfanylbut-1-ene 2e.